The coupling reaction of an alkyl halide and a Grignard reagent has been known for some time and, when catalyzed by a transition metal catalyst (which includes metal complexes and salts), is known as the Kharasch Reaction..sup.1,2,3 Unfortunately, both yield and selectivity are usually low and the reaction is suitable only for special cases. FNT .sup.1 Journal of Organometallic Chemistry, 42, 205,206 (1972), M. Tamura and J. Kochi. FNT .sup.2 Journal of Organometalic Chemistry, 31, 289 (1971), M. Tamura and J. Kochi. FNT .sup.3 Bulletin of the Chemical Society of Japan, 48, 2389 (1975), Y. Ohbe and T. Matsuda.
An allylic halide has been coupled with a benzylic halide employing the Wurtz reaction, but here again selectivity and yield has been low due to self-coupling (i.e., coupling of two molecules of the allylic halide) along with the desired cross-coupling (of the allylic halide with the benzylic halide). Disproportionation and hydrogen-abstraction have also contributed to lack of selectivity and low yield.
An added problem found in using allylic halides is that the cross-coupling can produce a mixture resulting from both direct displacement and displacement with allylic rearrangement. In the former, the halogen atom is simply displaced by the R group of the Grignard. In the latter, the R group attaches itself to the gamma carbon (with respect to the halide) of the allylic halide group with migration of the double bond in the direction of the halogen atom and loss of halogen.
Recently, advances have been made in the use of these coupling reactions for the synthesis of certain compounds. It is reported in Bull. Chem. Soc. Japan, 45, 2947 (1972) by Ohbe and Matsuda, with regard to the reaction of an allylic halide and an alkyl Grignard reagent in the presence of a transition metal salt, that three reactions occur competitively: (a) reduction of the allylic halide to the olefin, (b) cross-coupling of the halide and alkyl Grignard reagent, and (c) self-coupling of the allylic halide. It is indicated that in the case of allyl chloride, the ratio of cross-coupled product to self-coupled product ranges from about 83:17 to about 12:88. The coupling reaction was carried out employing ethereal solutions of the allylic halide and alkyl Grignard reagent.
Two other publications, Synthesis, 303 (1971), and Journal of Organometallic Chemistry, 42, 205 (1972), both by M. Tamura and J. Kochi, describe the use of tetrahydrofuran (THF) as a solvent for the coupling of Grignard reagents and alkyl halides. The coupling reactions are copper catalyzed, and it is disclosed that the organometallic intermediates are more stable in tetrahydrofuran than in conventional solvents. The publication does not deal with allylic halides, but simply points out that the reaction is mostly applicable to primary bromides and that "secondary and tertiary alkyl halides generally give poor yields of coupled products and disproportionation predominates." Under the Tamura and Kochi conditions, cross-coupling is by far the predominant reaction with the yield of self-coupled dimers being negligibly small.
More recently, Mesnard and Miginiac reported, [C. R. Acad. Sci., Ser. C., 277 (14), 567 (1973),] high selectivity for displacement with allylic rearrangement in the reaction of 1,4-dihalo-2-butenes (a primary allylic halide) and saturated Grignard reagents. The coupling reaction was carried out employing ethereal solutions of both reagents and no catalyst.
Besides employing different solvents in the above works, the authors of the three studies used significantly different experimental procedures. The Ohbe et al. method involved refluxing the ether-Grignard reagent solution with the metal catalyst for approximately one hour followed by rapid addition of the allylic halide to the solution and continued refluxing. The Tamura and Kochi method involved addition of the copper catalyst to a stirred and cooled (less than 0.degree. C.) solution of the alkyl halide and Grignard in tetrahydrofuran. The Mesnard and Miginiac procedure involved addition of the Grignard solution to the allylic dihalide at -10.degree. C. followed by stirring at ambient temperature for 24 hours.